Amplifier with negative feedback



Jan l0, 1950 H. G. BELJRS ETAL 2,494,426

AMPLIFIER WITH NEGATIVE FEEDBACK Filed July 2, 1946 AENZ Patented `an. 10,A 1950 AMPLIFIER WITH NEGATIVE FEEDBACK Hugo Gerrit Beljers, Willem Six, and Jan te Winkel, Eindhoven, Nethe ford National Bank rlands, assignors-t'o Hartand Trust Company, Hartford, Conn., as trustee Application July 2, 1546; Serial No.

In the Netherlands March 4, 1943 section 1, Public Lau1 690, August s, '1946 Patent expires March 4, 1963 2 Claims. (Cl. 179-171) This invention relates to an amplifier with l' negative feedback the amplification of which is variable with frequency. The desired variation vof amplification with frequency may be obtained -in known mannerv by including in the negative feedback circuit a network thetransmission factor ,8 of which in respect of its magnitude exhibits a variability with` frequency reciprocal to that of the desired amplification. In fact, for the amplification'p.' of an amplifier with negative feedback there applies the formula 1+'. in which u represents the -amplincatlon without negative feedback whichfor ,c1 changes to Lag 1 from which it appears that the amplification p. lis( reciprocal to the transmission factor of the network in the negative feedback circuit. Consequently, by constituting this network in a suitable manner it is possible to obtain any desired variation of amplification with frequency. However, the network will also produce a phase displacement variable with frequency in the negative feedback circuit and this detrimentally affects the stability of the amplifier, since in deite frequency ranges the negative feedback changes to positive feedback with the danger involved for self-oscillating of the amplifier.

The present invention purports to provide steps whereby the detrimental iniiuence of the network in the negative feedback circuit upon the phase displacement of the negative feedback voltage is avoided,

According to the invention, this object is achieved by including a network in the output circuit of an amplifying tube of the amplifier. which network is constituted in such manner that the amplification factor i, of the amplifying tube and the network together substantially satises the relation i=constant and real.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully by reference to the accompanying drawing.

Figure 1 shows a diagram of the principle of an amplifier according to the invention. This amplifier comprises an amplifying tube l coupled to output terminals 4, 5 through a network 2 and an amplifier 3. The voltage e1 to be amplified is supplied to input terminals 6, 1. The input circuit has also supplied to it a negative feedback voltage ek which is derived, through a network Cil ' grid and the cathode of yil, from the output terminals of the amplifier between which theoutput voltage e3 is set up. The amplication, factor of the amplifying. tube jl and the network 2 without negative feedback,

that is to say the relation which exists between Vthe voltage e2 and the voltage e between the the amplifying tube l is assumed as ,B1 and the amplification factor of the amplifier 3 63 u Consequently a #B51 c* 1+uf Now, according to the invention, the networks 2 and 8 are constituted in such manner that 1=1c=constant andreal, so that the abovementioned relation changes to C 6":.1-llL-uke 4from which it follows lthat the phase displacevment of the negative feedback voltage ek relatively to the input voltage e1 is invariable with the properties of the network 8. It will now always be possible for i to be made exactly constant.

, A very small phase displacement is already obtained, if i is maintained constant as far as possible.

In Figure 2 an impedance Z1 is substituted for the network 2 shown in Figure 1 and an impedance Z3 for the network 8. In series with Z3 is u connected, in addition, an impedance Z2. Z2 and Za are constituted by impedances Z2', and Z3', re-

spectively, which are shunted by a resistance R. For the negative feedback voltage eu applies. as before,

3 As before, between ek and e1 there exists a phase relation independent on the network in the negative feedback circuitif the product [3,31 is constant. Since this will be the case if ZiZ3=constant Z2+Z3 :constant-.24

Z1Za=constant implies thatthe impedances' Zi and Z3 must be inverse.

The second condition Zz-l-Zf--constant may-be- It willi not.alwaysbe` possible to. fulllY exactly .the-condition ZzZa, since-Z3 comprises the parallel resistancefR; which' makes itLnecessary for Z1 il toc` comprise a serieseresistance;

Now, Z1 comprises;alsostlie-capacity whichexists. between the .anode anditheccatho'de-eof: thealamplifying tube I Whatwe claim is:

" '1'. An am'plifyin'gosysftem fc'nelectricalr oscillations comprising a first amplifying stage including an electronvdischarge tube provided with l a cathode, a grid andia'n anode and an output impedance. Z1 connectedbetween said `anode vand factor variable with frequency, said network including a rst negative feedback impedance Zz connected across'the output of said second stage and a second negative feedback impedance Z3 interposedin series between the output of said secondrstageandsaid first feedback impedance,

.andmeans to apply said electrical oscillations in series with the feedback wave developed across saidfirstfedback impedance as an input to the fullled by constituting Z2 and Za mthemanner grid 'of said tube, said impedances having respective, values, substantially satisfying the relations.ZiZs"-'constant, and Z2+Za=constant- 2. An arrangement as set forth in claim 1 wherein impedance Z2 isconstituted by the parallel connection of an impedance having a value Zz `and-a"'rst resistance-havinga'value Ri and wherein ifnpedanceZav is constitutedby aniim` pedance havingavalue-Za" and a second'resi'st ance also having avalueR, the respectivevalues of f impedances Zzv and Zs' substantially 'satisfying the relation Z2 Z3"=R2 JANTEWINKEL'.

REFERENCES/1 Crrnllf,i

The following referencestare of record in the le of this patent:`

Number Name ate 2,196,844 West -Y Apr. 9, 1940 FOREIGN PATENTS Number CountryY Date Y 484,332, GreatBritaincnc-- Nov. 19, 1937 

